1. Field of the Invention
The present invention relates to a method for producing aluminum matrix composite materials.
2. Description of the Prior Art
There has been considerable effort over the past 10-15 years to develop aluminum-based alloys with improved specific strength and stiffness for use in advanced structural applications. One approach which has been explored is the addition of ceramic particles to produce a ceramic-reinforced metal matrix composite. Silicon carbide (SiC), in the form of whiskers or particulates, has generally been utilized as the reinforcement because of its commercial availability. Silicon carbide-reinforced aluminum matrix composites have been produced using a variety of processing methods. These methods include conventional solidification casting and powder metallurgy techniques in which the reinforcement particles and matrix alloy are blended together and subsequently consolidated together in the solid state.
One problem which limits the elevated temperature capability of silicon carbide-reinforced aluminum matrix composites is chemical interaction between the alumimnu matrix and silicon carbide reinforcement. This chemical interaction takes place during composite processing and subsequent thermal exposure in service. The interaction results in chemical degradation of the reinforcement with the loss of its desirable properties and the formation of various reaction products at the reinforcement/matrix interface of the composite material. These factors influence negatively the overall mechanical properties of the composite material. To overcome these problems a variety of techniques and composite materials have been developed. The prior art illustrates these attempts to integrate a variety of matrix materials with reinforcements.
In U.S. Pat. No. 5,084,088, entitled High Temperature Alloys Synthesis By Electro-Discharge Compaction, novel materials are formed by the combined action of high pressure and high current density acting on blended powders. The process may produce a titanium alloy with TiC and Ti.sub.4 B reinforcement by mixing Ti and B.sub.4 C powders. Additionally this patent discloses a method to produce non-reinforced alloys from tantalum, niobium, titanium and aluminum. The non-reinforcement process is similar to shock wave consolidation, in which a high pressure shock wave is passed through a powder pack. In both processes, the energy is concentrated at the particle/particle boundaries, where it causes melting of the powders in the near-boundary regions of the powder pack. With the applied pressure, this results in densification as well as chemical interaction to form new phases. However, one drawback of this process is that the reaction products are formed only in the particle/particle boundary regions, where the melting and thus the alloying takes place. Very little change occurs at the interior region of the powder particles. Although this phenomena is identified as an advantage in the patent merely because the material appears to be homogeneous on a macroscopic scale, the material is less than adequate because it is actually quite nonuniform on a microscopic scale.
U.S. Pat. No. 4,808,373 is entitled an In-Situ Process for Producing a Composite Containing Refractory Material. The patent describes a process for producing a composite material consisting of a refractory material dispersed in a matrix of aluminum, copper and nickel, beryllium, magnesium or a ceramic material such as silicon dioxide. The refractory material is an interstitial compound, such as a carbide, boride, or nitride phase. A refractory-forming component (a reactive metal such as tantalum) is combined with a reactive component such as carbon (contained in a gas phase), to form the refractory carbide reinforcement phase. A compound such as tantalum aluminide may also be introduced to the refractory--forming component tantalum in the system. During the chemical reaction that follows, the aluminide phase undergoes chemical reduction to form a carbide phase resulting in free aluminum. In this type of process, the aluminum matrix may act as a carrier of one or both of the reactants which form the reinforcement phase, but the matrix does not take part directly in the reaction itself. In addition, the reinforcement phase formed is a carbide, rather than an aluminide. The reaction takes place between refractory-forming elements such as tantalum and reactive components such as carbon while the matrix is present merely as a carrier.
U.S. Pat. No. 5,061,323 is entitled Composition and Method For Producing an Aluminum Alloy Resistant to Environmentally-Assisted Cracking. This patent describes a process for making an aluminum alloy containing molybdenum particles. The process is entirely a solid state process and no melting takes place. The process requires extraneous processing steps of extrusion to achieve good particle bonding and optimum mechanical properties.
U.S. Pat. No. 5,926,574 ('574) is entitled Molybdenum Based Substrate Coated with Homogeneous Molybdenum Trialuminide. This patent discloses a process of coating a molybdenum substrate by using a chemical reaction with the substrate to form the coating "in-situ". The term `composite` used in this patent has a different meaning than used in the context of the previous patents. Here composite is taken to mean a metal substrate with a coating material rather than an integral reinforced metal matrix material. This process results in an oxidation resistant coating for molybdenum provided by the molybdenum aluminide coating, but does not improve the structural properties of either the aluminum or the molybdenum.
U.S. Pat. No. 4,402,744 entitled Chemically Bonded Aluminum Coating for Carbon Via Monocarbides, is similar to the '574 patent in that the process described results in a coating on a substrate, rather than a structural composite material.
U.S. Pat. No. 5,059,490 is entitled Metal-Ceramic Composites Containing Complex Ceramic Whiskers. The process and material are exemplary of the so--called "XD" process developed by Martin Marietta Inc. Other "XD" patents or variants thereof include U.S. Pat. Nos. 4,751,048, 4,774,052, 4,836,982, 4,915,905, 4,916,964, 4,917,964, 4,985,202, and 5,015,534. These processes require an additional melt step to introduce the intermediate material (a porous sponge) into the final matrix in order to form the reinforcement. The matrix in the XD process acts only as a solvent or host for various reactants and does not take part directly in the reaction to form the reinforcement phase. The reinforcement phase identified in aluminum matrices is never an aluminide or any other aluminum compound. Thus one cannot form an aluminide reinforcement in aluminum using the XD process.